Researchers at the University of Cambridge have demonstrated an “artificial leaf” that uses only sunlight, carbon dioxide and water to produce syngas. As a a result, unlike the current industrial processes for producing syngas, the leaf does not release any additional carbon dioxide into the atmosphere. The results are reported in the journal Nature Materials.

Architecture of the standalone perovskite–BiVO₄ PEC tandem device for bias-free syngas production. Oxygen evolution occurs at the front BiVO₄ photoanode with a Co WOC. An inverse-structure perovskite photocathode reduces CO₂ and protons to CO and H₂ via a CoMTPP molecular catalyst immobilized on a CNT sheet. An embedded copper wire connects the two photoelectrodes in this artificial leaf configuration. FTO, fluorine-doped tin oxide; PCBM, [6,6]-phenyl C₆₁ butyric acid methyl ester; PEIE, polyethylenimine. Andrei et al.

Syngas is currently made from a mixture of hydrogen and carbon monoxide, and is used to produce a range of commodities, such as fuels, pharmaceuticals, plastics and fertilizers.

Syngas, a mixture of CO and H₂, is a crucial intermediate in the industrial production of methanol, higher alcohols, long-chain hydrocarbons, lubricants, waxes and fuels via the Fischer–Tropsch process. … The conventional reforming of methane to syngas relies on fossil fuels to operate at high temperatures and pressures and biomass gasification can introduce contaminants. The solar-driven production of syngas from aqueous CO₂ is an ambient conditions and clean alternative process. Although silicon, dye, metal oxide and perovskite photoabsorbers provide enough driving force in tandem devices for bias-free water splitting, very few examples of bias-free photoelectrochemical (PEC) CO₂ reduction are known.

Owing to the large overpotentials that need to be overcome for a simultaneous CO₂ reduction and water oxidation, most of those systems employ up to six photovoltaic (PV) solar cells connected in series or complex nanostructures based on noble metals. Accordingly, a vast library of molecular catalysts that employ earth-abundant metals for CO₂ reduction remains underexplored. Such catalysts are known to achieve improved selectivities towards CO production at lower overpotentials—Co porphyrin and phthalocyanin recently demonstrated a selective aqueous CO₂ reduction to CO when immobilized onto carbon nanotube (CNT)-based electrodes.

Here we tap into that library of molecular catalysts by using the commercially available cobalt(II) meso-tetrakis(4-methoxyphenyl)porphyrin (CoMTPP), which can be readily immobilized via π−π stacking interactions onto CNT sheets, also known as buckypaper. The composite is employed in electrodes, state-of-the-art perovskite-based photocathodes and perovskite–BiVO₄ PEC tandem devices, which couple tunable syngas to O₂ production in an aqueous solution.

—Andrei et al.

You may not have heard of syngas itself but every day, you consume products that were created using it. Being able to produce it sustainably would be a critical step in closing the global carbon cycle and establishing a sustainable chemical and fuel industry.

—senior author Professor Erwin Reisner from Cambridge’s Department of Chemistry, who has spent seven years working towards this goal

Andrei et al.

On the artificial leaf, two light absorbers, similar to the molecules in plants that harvest sunlight, are combined with a catalyst made from the naturally abundant element cobalt.

When the device is immersed in water, one light absorber uses the catalyst to produce oxygen. The other carries out the chemical reaction that reduces carbon dioxide and water into carbon monoxide and hydrogen, forming the syngas mixture.

As an added bonus, the researchers discovered that their light absorbers work even under the low levels of sunlight on a rainy or overcast day.

This means you are not limited to using this technology just in warm countries, or only operating the process during the summer months. You could use it from dawn until dusk, anywhere in the world.

—Virgil Andrei, first author

The research was carried out in the Christian Doppler Laboratory for Sustainable SynGas Chemistry in the University’s Department of Chemistry. It was co-funded by the Austrian government and the Austrian petrochemical company OMV, which is looking for ways to make its business more sustainable.

Other artificial leaf devices have also been developed, but these usually only produce hydrogen. The Cambridge researchers say the reason they have been able to make theirs produce syngas sustainably is thanks the combination of materials and catalysts they used.

These include state-of-the-art perovskite light absorbers, which provide a high photovoltage and electrical current to power the chemical reaction by which carbon dioxide is reduced to carbon monoxide, in comparison to light absorbers made from silicon or dye-sensitised materials. The researchers also used cobalt as their molecular catalyst, instead of platinum or silver. Cobalt is not only lower-cost, but it is better at producing carbon monoxide than other catalysts.

Syngas is already used as a building block in the production of liquid fuels. The team is now looking at ways to use their technology to produce a sustainable liquid fuel alternative to gasoline.

What we’d like to do next, instead of first making syngas and then converting it into liquid fuel, is to make the liquid fuel in one step from carbon dioxide and water.

—Erwin Reisner

Although great advances are being made in generating electricity from renewable energy sources such as wind power and photovoltaics, Reisner says the development of synthetic gaosline is vital, as electricity can currently only satisfy about 25% of our total global energy demand.

We are aiming at sustainably creating products such as ethanol, which can readily be used as a fuel,” said Andrei. It’s challenging to produce it in one step from sunlight using the carbon dioxide reduction reaction. But we are confident that we are going in the right direction, and that we have the right catalysts, so we believe we will be able to produce a device that can demonstrate this process in the near future.

—Virgil Andrei

The research was also funded by the Winton Programme for the Physics of Sustainability, the Biotechnology and Biological Sciences Research Council, and the Engineering and Physical Sciences Research Council.

Resources

• Virgil Andrei, Bertrand Reuillard and Erwin Reisner (2019) “Bias-free solar syngas production by integrating a molecular cobalt catalyst with perovskite-BiVO₄ tandems.” Nature Materials doi: 10.1038/s41563-019-0501-6